This invention relates generally to a voltage controlled oscillator (VCO) circuit adapted for use in phase locked loop operation and more particularly to a VCO circuit that comprises a current mirror circuit section connected to an oscillator circuit section a provides an output having a selected frequency based upon the current output of the mirror circuit coupled as an input to the oscillator circuit and, in addition, provides means to control mirror circuit output current.
The phase locked loop (PLL) circuit is a circuit of the frequency feedback type comprising a phase comparator, a low-pass filter, an error amplifier, and a voltage controlled oscillator (VCO), which is adapted to generate a desired frequency in accordance with an externally applied signal. As well known in the art, the VCO is a circuit component indispensable to the PLL circuit.
FIG. 1 is an equivalent circuit diagram of a conventional VCO circuit employed in a PLL circuit, for example, the VCO circuit employed in chip MM74HC4046 manufactured by National Semiconductor Co., Ltd. Connection points 21, 22, 11 and 12 in FIG. 1 as well as in FIG. 3 are respectively pin terminals numbered 6, 7, 9, and 11 for this particular chip MM74HC4046 . This equivalent circuit is composed of a current mirror circuit section 10 and an oscillator section 20. To obtain an output current, I.sub.1, from current mirror circuit section 10 corresponding to a desired frequency characteristic, a resistor 13 is connected between pin terminal 12 of the current mirror circuit section 10 and ground. Further, a capacitor 23 for restriction of the oscillating frequency is connected between pin terminals 21 and 22 of oscillator section 20.
With resistor 13 and capacitor 23 connected as described above, a control voltage, V.sub.D, is applied to input 11 at pin terminal 9 of the VCO circuit to selectively vary the frequency output thereof. As a result, the output current I.sub.1 corresponding to the input control voltage, V.sub.D, is applied from current mirror circuit section 10 to oscillator circuit section 20 as a control current and oscillator circuit section 20 oscillates at a frequency corresponding to control current I.sub.1. Therefore, an oscillating frequency signal having a desired frequency characteristic is provided as an output terminal 24 of the VCO circuit.
FIG. 2 is a frequency characteristic graph for the VCO circuit shown in FIG. 1. As will be appreciated from FIG. 2, the oscillating frequency, f.sub.rco varies depending on the capacitance, C, of capacitor 23 and the resistance, R, of resistor 13 and provides a variable frequency range on the order of 10dB, extending from a few MHz to several tens of MHz. Thus, it can be seen that the range of frequency variation is quite narrow. To expand the variable range of frequency, both capacitor 23 and resistor 13 must themselves be changed, leading to the problem of requiring another circuit or other components having different values.
It is an object of the present invention to provide a VCO circuit capable of selectively extending the range of frequency variation by the addition of a simple circuit configuration.